Method and means for measuring and correcting delay and attenuation in a transmission channel



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0cm i 1968 w. o. CANNON ,333 METHOD AND MEANS FOR MEASURING AND CORRECTING DELAY AND V ATTENUATION IN A TRANSMISSION CHANNEL Filed Feb. 24, 1965 ll Sheets-Sheet 11 ATTORNEY United States Patent 3,404,338 METHOD-AND MEANS FOR MEASURING AND CORRECTING DELAY AND ATTENUATION IN A TRANSMISSION CHANNEL William D. Cannon, Metu chen, N.J.,-' assignor to The Western Union Telegraph Company, New York, N.Y., a corporation of New York Continuation-impart of application Ser. No. 13,866, Mar. 9, 1960. This application Feb. 24, 1965, Ser. No. 439,512

' 17 Claims. (Cl. 32457) ABSTRACT OF THE DISCLOSURE A method and means is disclosed for rapidly measuring relative delay and attenuation in a transmission circuit over which frequency-modulated waves are transmitted. Upon reception of the transmitted modulated carrier wave, measurements are made of frequency of the transmitted carrier, and of attenuation during transmission. The received modulated carrier wave is detected, the modulating wave is recovered, filtered, squared and then compared with a locally generated reference wave to obtain a measurement of delay in transmission. Frequency-attenuation and frequency-delay characteristics are displayed by curve tracing means using the obtained measurements. Adjustable networks are also provided to compensate and cor rect for irregularities in attenuation and delay while the extent of compensation and correction are monitored by the curve tracing means.

This application is a continuation-in-part of my copending US. application Ser. No. 1 3,866, filed Mar. 9, 1960, now abandoned, which was in turn a continuation-in-part of my copending US. application Ser. No. 621,476, filed Nov. 9, 1956, now abandoned, which was in turn a continuation-in-part of my copending U.S. application Ser. No. 469,948, filed Nov. 19, 1954, now abandoned.

The present invention relates to communication circuits such as are used'for the transmission and reception of facsimile images, very high-speed telegraph signals, other varieties of high speed digital or pulse communication, and to the compensation of the distortion occurring is such signals due to the'nonlinear attenuation and delay characteristics of the transmission path.

More particularly, the invention concerns procedures and means for recognition and measurement of group or envelope delay occurring in the transmission of modulated signals as a means of investigating the transmission delay characteristic of a transmission path, and for the compensationof this characteristic. The invention contemplates alsothe measurement of the attenuation characteristic of the transmission pathand the compensation of any irregularities occurring therein.

The invention is for a unique method for measuring envelope delay over transmission lines, circuits, or devices of any kind and for whatever purpose so long as this purpose lies 'within the practicable frequency rang of the method. In particular, high-speed pulse transmission types of signals whether sentdirectly, or modulated on a carrier frequency are susceptible to distortion as a consequence of any nonlinearity in the delay characteristic of the transmission path, device or network. Facsimile signals and signals for very high-speed-telegraphy,such as are encountered in multichannel telegraph systems and in systems for the transmission of data at high-speed between business machines, areexamples of the type ofsignal to which the invention is particularly applicable. The invention also utilizes unique compensating networks for the correction of the measured nonlinearities or. irregularities in both the transmission delay and the attenuation and sets forth a highly expeditious correction procedure utilizing the measuring instruments and the compensating networks described herein for the purpose of eliminating signal distortion. The unique networks are the subject of a copending application Ser. No. 434,302, filed Feb. 23, 1965, and now Patent No. 3,400,329.

Transmission delay is basically the time elapsing between the application of a transmitted electrical signal to the transmitting terminal of a circuit or device under test and its receipt at the distant output end of the test circuit. In one oft-proposed method for measuring this time lapse, the receiving end receives information as to the departure time of a test signal via a separate auxiliary path or channel having a known delay and this information may be compared with the arrival time of the test signal for the purpose of determining the transmission time over the circuit under test. In practice, however, even if such an auxiliary channel is available its characteristics may not be accurately known and they may be obscured by noise, distortion or instability and so reliable measurement is impossible. In an alternative method, the received test signal is compared with a locally generated reference signal of the same general character which should be of a very high order of stability. This method avoids the need for an auxiliary circuit but in the past it has not produced satisfactory results because of failure to appreciate the need for extraordinary stability in the reference signal, riddance of the eliects of interference and distortion on the received test signal, and the implementation of a highly accurate and stable comparison method together with simplified, accurate and readily manipulated delay indicating means.

Applicant, by perceiving the shortcomings and the relative practical and economic factors present in the two approaches to transmission delay measurement, has adopted the latter method and has so perfected the individual and cooperative functioning of the elemental processes by which the delay measurement is ultimately registered that the measurement can be readily and accurately made by relatively unskilled personnel. Furthermore, the measuring instrument is readily portable and, by dispensing with the auxiliary circuit extraordinary flexibility is achieved.

It is accordingly an object of the present invention to provide a compact unitary equipment for the rapid and convenient measurement of the delay characteristic of communication lines, networks or devices.

Another object is to provide means and a method for a greatly simplified technique for measuring signal delay.

A further object is to provide a continuous display of an overall delay characteristic in a condition suitable for viewing or for photographic recording.

Still another object of the invention is to provide means for the simultaneous measurement of the delay and amplitude characteristic of a transmission line or device.

A primary object of the invention is to provide novel means for the compensation of nonlinearity or irregularity in the transmission delay characteristic of a line or device.

A further object is to provide simultaneous compensation of both amplitude and delay characteristics by means of a single device.

A further object is to present visually the delay characteristic of a transmission line or device Over either its entire frequency range or over specified parts thereof.

A further primary object of the invention is to provide a method of measuring transmission delay directly in units of time.

A still further object is to provide means for measuring relative delay of lines where the terminals are isolated from each other.

A prominent object of the invention is to provide means for measuring the relative transmission delay of a long 3 line without the necessity of an auxiliary path between the two ends of the line for the purpose of comparing a received test signal and a reference signal.

A further primary object of the invention is to provide means for measuring relative transmission delay through linear comparison of a received sine wave test signal and a reference signal of substantially the same frequency but having only random phase relation thereto.

Another object is to provide means for measuring relative transmission delay by comparing a received test signal and a reference signal, both having nominally the same but not necessarily synchronous frequencies.

A specific object is to achieve linearity of the comparison between the sine wave test signal and the reference signal by effectively making the comparison at the positive and negative peaks of the sine wave test signal.

Another specific object is to provide means for measuring transmission delay by comparing a received test wave and a reference wave at two cardinal points per cycle.

A principal object is to measure transmission delay over circuits or transmission lines with accuracy and dependability despite the presence of noise, attenuation variation and signal distortion on the transmission media under test.

Other objects and advantages of the present invention will appear upon the consideration of the following detailed description of the invention and the drawings, wherein;

FIG. 1 represents phase shift and delay curves for a line or device having delay linear with frequency;

FIG. 2 represents phase shift and delay curves for a line in which these properties are not linear with frequency;

FIGS. 3a and 3b represent the envelope of a detected wave composed of two slightly separated component frequencies, the wave of FIG. 3b being composed of a 50% modulated carrier frequency to produce a carrier and two first order side band frequencies;

FIG. 4 represents the time relationships which exist between signal and reference pulses as applied forcomparison purposes to the two grids of a flip-flop circuit;

FIG. 5 discloses the elements of three different basic types of delay compensating networks;

FIGS. 6 and 7 illustrate the circuitry of two practicable network circuits for delay compensation;

FIGS. 8, 9 and 10 are reproductions of photographs of oscilloscope curves representing the delay and amplitude characteristics of typical lines before and after compensation by means of the networks of this invention;

FIG. 11 gives details of the sweeping oscillator section of a beat frequency oscillator used to provide the variable carrier frequency employed for making delay measurements;

FIG. 12 is a schematic wiring diagram of a circuit for correcting irregularities in transmission delay according to the instant invention;

FIG. 13 is a block diagram of an instrument'for measuring transmission delay in accordance with the instant invention;

FIG. 14 is a schematic wiring diagram of the transmitter portion of a transmission delay measuring instrument comprising FIG. 13; a

FIG. 15 is a schematic wiring diagram of a frequency measuring circuit for use as a part of the measuring instrument comprising FIG. 13;

FIG. 16 is a schematic wiring diagram of a detector circuit for use as a part of the measuring instrument comprising FIG. 13; I

FIG. 17 is a schematic wiring diagram of a limiting and pulse forming circuit for use as a part of the measuring instrument comprising FIG. 13;

FIG. 18 is a schematic wiring diagram of a delay measuring circuit for use as a part of the measuring instrument comprising FIG. 13;

FIG. 19 is a diagram illustrating the method of com: bining FIGS. 15, 16, 17 and 18 for circuit continuity;

FIG. 20 is a schematic wiring diagram of another embodiment of compensating device for use in the assembly of FIG. 12;

FIG. 21is-a wiring diagram'of an alternative portion of FIG. 20; and i FIG. 22 is a schematic: wirin'gdiagram of a different embodiment of the delay measuring-circuit fromthat showninFIG. 18. r t i Signals for, electrical communication are, 'in general, transient or repetitive and, asis well known', these transients or repetitive pulses may be resolved into a fundamental frequency and a series of,higher harmonics thereof. In a square wave, for example, when transmitted, these frequency components all bear a definite relationship to each other in respect to.-.amplitude and phase, and it is desired that the same wave when received resemble its original configuration within specified limits. However, the attenuation of the transmission medium may vary over the range of these frequencies with the consequence that the shape of the received signal will depart from' its original form. Likewise, the transmission time delay may vary over the range of frequencies so that the different frequency components reach the receiver at different instants of time, with the consequence that a still different form of distortion may occur. A study of these types of distortion as produced by transmission media is found in an article by S. P. Mead, entitled Phase Distortion and Phase Distortion Correction, in the Bell System Technical Journal, page 195, volume 7, 1928, and in a second article entitled Measurement of Phase Distortion, by H. Nyquist and S. Brand, found on page 522 of the same publication, volume 9, 1930.

Proceeding with the examination of transmission delay and the application of the present invention to its measurement and compensation, it is observed that signals involving a relatively wide transmission band, in terms of octaves, are particularly liable to this type of distortion. One exception is the case of telephony where, although the distortion occurs, the reception of telephone sounds is not seriously impaired thereby because of the tolerance of the human ear to this type of distortion. Facsimile signals on the other hand are a type of pulse transmission and are particularly subject to delay distortion especially since, due to the inherent inefiiciency of this mode of transmitting intelligence by known methods, it is customary to exploit the available bandwidth of the transmission medium to the fullest extent. The signals, ranging from large areas of solid black on the one hand, to fine'lines on the'other hand, comprisea frequency range of several octaves and hence if nonlinear transmission delay is present in the line or other transmission medium, the received facsimile image will suffer loss ofdetail, echoes, and other impairment to fidelity of reproduction.

Modern telegraph systems are reaching to higher speeds as the number of channels is increased in multiplex type systems, and also in some types'of single channel systems designed to accommodate the high-speed output of business machines. These effects of nonlinear delay in the transmission medium upon the transmission ,of both facsimile and telegraph signals are serious whether or not the signals are transmitted directly as pulses were modulated on a carrier frequency, but they are particularly seious in the case of direct pulse transmission. In either event, for best utilization of the transmission medium, device or network, it is necessary to identify, measure and compensate to within tolerable limits any nonlinearities in transmission'delay.

One method which' has' been commonly usedfo'r measuring transmission delay is-"to sen'd-a sinusoidal signal of adjustable frequency over the'line, device or network under measurement while sending, as'a' reference, a second signal of like frequency over .asecond'and distortionless line connecting the same termini and then measuring the phase relationship of the two frequencies at the receiving end. Alternatively, a fixed frequency derived from the same base generator as the measuring frequenciesmay be sent over the auxiliary channel and various reference frequencies then derived therefrom at the receiving end. By thus measuring delay at a series of frequencies of interest, successively, a plot of the overall transmission delay characteristic can be constructed. It is apparent that such a method is very laborious and risks overlooking intermediate peak aberrations unless the points of measurement are very closely spaced. It is evident further that the conduct of corrective procedures, if dependent upon such a method of measure ment, are exceedingly tedious and time consuming. A primary objection to such a method is the requirement of a reference standard of phase transmitted from the sending end of the circuit. Even if a separate channel for this purpose is available, such channel is subject to distortions of its own and if either the circuit under measurement or the auxiliary channel involves carrier transmission, then either or both are subject to frequency aberrations due to nonsynchronism of the pairs of translating oscillators of the carrier system terminals. All of these difiiculties have been overcome in the system to be described.

FIG. 1 is illustrative of a typical delay characteristic of a transmission line. In this case, delay is constant with frequency and signals transmitted over such a line will not be distorted. The slight line phase characteristic, it is noted, terminates at the origin. In the event that the phase characteristic does not pass through the origin as indicated by the dash line, in certain circumstances distortion may occur, but in most cases such distortions are of little consequence and are not dealt with further in this specification.

FIG. 2 is similar to the preceding FIG. 1, but the delay and phase curves are illustrative of a line possessing nonlinear delay. The aforementioned paper by Nyquist and Brand examines the case of nonlinear delay and suggests a method of delay measurement. If fl/w, where 5 represents phase shift and w is proportional to frequency, represents phase shift and w is proportional to. a line, then dfi/dw, the first derivative thereof, represents a quantity termed the envelope delay for the line. This quantity plays a fundamental :part in determining the delay of a system and has the advantage that it is constant for a distortionless system whereas 5/ varies with frequency. It is determined further that envelope delay may be measured directly by transmitting over a line a carrier fequency modulated by a relatively low frequency so that, in effect, the two first order side bands of the transmitted wave comprise two slightly separated frequencies of equal amplitude. The modulated wave envelope suffers no significant distortion regardless of the length of line. For practical purposes, measurement of the delay of this envelope as the pair of sideband frequencies is varied over the frequency range of interest represents the envelope delay of the line.

Under the present invention a sinusoidal carrier of continuously adjustable frequency is amplitude modulated by a signal of relatively low frequency, and this wave, consisting only of the carrier and two first order sidebands, all having nearly the same high frequency, after transmission ove the line, is demodulated at the receiving end to regenerate the modulating frequency. By providing, as a reference of comparison, a local generator operating independently of, but at approximately the same low frequencyas the modulating frequency a determination of the difference in time of transmission of the regenerated modulated wave for various carrier frequencies is accomplished. This is done by measuring the time interval between the occurrence of correspondingcritical points on the locally produced and the regenerated low frequency waves at one carrier frequency, and comparing this interval with that observed at other nearby carrier frequencies.

It can be shown that either amplitude or frequency modulation can be used to derive the three required frequencies. However, at the frequencies involved here, amplitude'modulation allows more simple instrumentation and the resulting amplitude modulated signals are more nearly analogous to those used in facsimile and telegraph transmission.

A significant feature of the invention is that the phase relationship which happens to exist betweenthe original modulating wave and the locally generated low frequency comparison wave at the moment of measurement does not affect the accuracy of the measurement, since only time, and not phase, differences are compared. The resulting determination is obtained by direct measure ment of time, and for this reason it is unnecessary to lock the local generator into phase synchronism with the transmitted signal, and in fact, a moderate amount of drift in the phase of the locally generated wave can be tolerated during the measurement, provided that it is small in relation to the quantities being measured.

A further advantage attributable specifically to this method of measurement is that delay determinations so made are measured in a direct and linear manner suitable for precise indication or recording, and not in a nonlinear manner such as occurs with phase comparison methods of measurement.

As a result of the wide tolerance of the measuring apparatus toward phase changes of the locally generated comparison Wave, one of the important and distinctive advantages of this system arises, namely, that the local generator is independent and free running, and need not be synchronized with the transmitter in either phase or frequency, being required to maintain only a close ap proximation of the modulating frequency, such as is readily obtained from commercially available signal generating sources. This renders unnecessary the transmission of synchronizing information to the receiving apparatus during the measurement, and results in greatly simplifying the equipment by the elimination of the apparatus components and the auxiliary channel associated with the production and use of such synchronizing signals.

Although the modulating frequency recovered from the demodulated signal is, as stated, a very low frequency, the time delay which it experiences is of the same order as that associated wtih the much higher frequency of the carrier wave. This is very advantageous for practical reasons, because changes of group delay corresponding to a carrier phase change of many radians if measured at the carrier frequency level is measured by a time delay of lessthan one repetition period, or half cycle of the modulating frequency so that, except for very long delays, no cyclical ambiguity is present in the measurement of delay distortionwhen made by the instant method of measuring the delay of the regenerated modulating frequency wave.

My invention concerns itself with the measurement of relative delay rather than absolute delay as being the factor with which we are primarily concerned in the reception of distortionless signals. It is obvious that if all of the components of a signal are delayed alike, the original wave shape will be preserved. The invention, therefore, concerns itself primarily with the measurement of the delay discrepancies which exist among the various components of the signal. Phase delay of transmission media can produce a variety of different forms of distortion involving not only the shift of the cross-over points of harmonic components but also the production of spurious harmonics. However, I have determined that the method of the invention, which provides a measurement of envelope delay also provides a satisfactory measure of phase delay, and that when compensation is carried out as directed by these measurements, that substantially distortionless transmission of the'signals follows.

If two sinusoidal waves of slightly different frequencies are propagated over a line having nonlinear delay, they will travel at different velocities and will form crests and troughs which will travel at a slightly different velocity from the component waves. The crests thus formed are delayed by the factor dfi/dw, commonly referred to as the group or envelope delay whereas the two components are each delayed by the factor 3/01. For a distortionless medium, the envelope delay and the phase delay become equal. These observations still apply when three frequencies of an amplitude modulated wave comprising a carrier and two side frequencies are transmitted, and the delay to the modulating wave is still dB/dw. The most important factor to be considered in general is envelope delay dfi/dw, and it is not necessary to consider phase shift directly as such. Although a line may not be entirely free of distortion and hence B/w does not equal dB/dw, it is usually found that when satisfactory correction has been made for envelope delay distortion, then phase delay is also within tolerable limits.

A wave exhibiting the crests and troughs typical of the transmission of two slightly different frequencies over a medium having phase distortion is indicated in FIG. 3a. The same wave, produced by 50% amplitude modulation of a carrier, is depicted in FIG. 3b. This is the type of signal which I employ for the measurement of relative delay. It comprises two closely spaced side band frequencies and an intermediately spaced carrier at substantial level which may be selected for frequency and level indicator purposes at the receiving end of the medium. After taking a delay reading at a first carrier frequency, a second reading is taken but with a carrier frequency differing therefrom by A). The difference is relative delay dfl/dw, at the carrier frequency.

A simple but extremely effective measuring technique is used for reading delay which comprises the comparison of the time position of the low frequency wave envelope derived from a received modulated wave, with a like low frequency reference wave having substantially the same frequency but of random phase. This independence of initial phase position is an important advantage of the invention rendered possible by the choice of comparison circuitry. The ultimate comparison device is a two-tube balanced bistable flip-flop wherein critical points of the received and reference waves are applied, respectively, to the grids of the two tubes and an integrating type DC meter is connected in bridge relationship across the two cathodes. The waves under comparison are applied to the flip-flop grids as uniform sharp pulses which in this case correspond to the zero or cross-over points of the original wave. To assure sharpness and fidelity of spacing of these pulses, the conversion from the original sine wave is accomplished by a square wave converter and limiter followed by a peakproducing differentiating circuit.

The reason that the bistable flip-flop and integrating meter, with pulse excitation of the grids, has unique application in this instrument is that the meter measures the asymmetry of the alternating dwell intervals of the two halves of the flip-flop, and this asymmetry is a linear measure of the time difference between the two low frequency waves. It is evident that if the exciting pulses applied to the two grids alternate with each other at uniform spacing then the meter must read zero, while departure from symmetry of spacing in either the plus or minus direction will cause an appropriate deflection on the meter.

The ultimate comparison device referred to is found at Q in the overall block diagram, FIG. 13, and in detail in the flip-flop tube 207 of FIG. 18; while FIG. 4 is an explanatory aid for the flipflop and meter operation. Reference to FIG. 4 will illustrate why when pulses corresponding to critical points of the reference envelope waves are applied to the right hand half of tube 207, while like pulses derived from the incoming envelope wave are ap- 8 plied to the left hand half of tube.207, the meter -M connected in bridge relation across the cathodes of the two halves of tube 207 will provide a linear indication of the discrepancies of time occurrence of the alternating critical points of the two envelope waves. In FIG. 4, and for getting for the moment the outlined sine waves, at (a) the series of negative pulses shown, occurring at the rate of 50 pulses per second, and produced at alternating cross-over points by the pulse former P of FIG- 13 represent the reference wave. The time position of this wave may be regarded as constant, against which the varying time positions of the incoming waves are to be compared. At (b), (c) and (d), pulses derived from an incoming 25-cycle wave, after having been subjected to differing delays, are depicted. It will be noted that the received pulses (produced by the pulse former M of FIG. 13 as will later appear) occur 50 times per second and so are directly comparable with the pulses of the reference wave.

At (b), the received wave is delayed or 10 milliseconds, and it is evident that since the intervals L and R between cross-over periods of the reference wave and received wave, respectively, are all equal, then the average voltage across the meter must be zero and for this condition the meter will read zero. This follows from the fact that application of a negative pulse to the grid of one tube of a bistable flip-flop establishes space current flow, or dwell, in the opposite tube. By adjustment, either of meter biasing circuits not shown, or by adjustment of a phase shift network in the circuit ofthe reference wave (such as that shown at 35.4 of FIG. 22) the meter can be set at zero for the beginning of a series of measurements. For the moment assume that the meter is of the zero center type. Now if the received wave encounters delay less than 90, it will move in the direction indicated at (c). As will later be described this produces a preponderance of dwell of the righthand half of tube 207 (FIG. 18) and the meter, so connected as to be deflected in the left hand or negative direction, will indicate a delay change of minus 5 milliseconds. On the other hand, assume that as the measuring carrier frequency is swept over the range, the amount of delay increases and the received wave moves in the direction shown at (d) in comparison with the reference wave. The left hand half of tube 207 now experiences a longer dwell and the meter will move to the positive portion of its scale, to indicate delay of plus 5 milliseconds.

Since ambiguity exists at the two full scale readings, -it is generally desirable to choose conditions of measurement such that extremes of the scale are not reached. This factor will be considered later in the discussion of the meter circuitry of FIG. 18. If desired, at the beginning of a series of measurements the meter reading can be brought to zero by an initial shift of the random phase relation of the reference wave. This, then, is the reference level of delay for the beginning carrier frequency located as desired, say at the lower edge or at the center of the band under measurement. Then, as the carrier is swept over the band, the delay is measured for each frequency relative to the arbitrarily chosen beginning frequency and delay reference level. The adoption of a delay reference level is entirely a matter of convenience because with the disclosd method the measurement is linear whether the asymmetry is large or small. Precise identity of the two low frequencies, their synchronism or their initial phase position are unimportant Due to the stability of practically available fork generators, the short time drift between the received and reference frequencies does not infiunce the readings. w

A number of meter adjuncts are possible. By rneans'of a suitable biasing circuit,-the zero point of the'meter, which would normally lie at mid-scale, can be moved to 'one end of the scale. Also, as previously mentioned, the maxi-. mum range of the meter is equal to the'time length of'one half-cycle of the wave envelope which in the case of 25 cycles is 20 milliseconds and, by additional circuitry, this range has been extended. By means of an additional stage of amplification, a meter bearing an expanded scale is also accommodated.

As previously stated, in this instrument the phase comparison is reduced to a measurement of time, that is, the intervals between discrete, short marker pulses, and hence is virtually independent of imperfections in received wave shape and noise as well as momentary synchronism and phase relation between .the received and reference frequencies. Compare these basic advantages with the limitations of prior art systems which relied upon devices for comparing the phase of sine waves with their requirement of vector addition with its nonlinear relationship between phase and time so that the two waves must not only be in synchronism but the reading devices must either compensate for the nonlinear phase-time relationship or supplemental calculations must be made.

Returning to FIG. 4, sine waves have been drawn into the rows of pulses (a), (b), (c), (d), in part to clarify the relationship of the reference and received waves and also to illustrate the problem which would present itself if the two waves were directly compared in some type of differential phase sensitive device as is customary in other types of delay measuring instruments. Obviously for a given degree interval, or time period, a sine wave undergoes a different amplitude change depending upon the position of the chosen interval along its course. For example, beginning at 0 on a sine wave, a 10 interval represents an amplitude change of 17% Whereas at 90 the same interval represents only a .016% change. Consequently, in this type of phase comparer, only like parts of the changing received wave may be compared with the identical part of the reference wave; hence the requirement for continuous synchronism of the received and reference waves in these earlier types of instruments.

In contrast, the extraordinary utility of this invention is apparent from the fact tha time intervals rather than wave amplitudes are being compared and it is of no consequence whether the two Waves are at crest or trough or any other position with respect to each other. This avoidance of a need for synchronism and the maintenance of a known phase position lead to the design of a relatively low cost instrument for measurement of time delay of lines of any length and with isolated terminals whereas in the past such measurements were laboriously made only with great difiicult and expenditure of time by highly skilled personnel using a costly assemblage of equipment. The manyfold increase in speed of measurement permitted hereby especially adapts the instrument for corrective procedures using my novel delay compensating networks described elasewhere in this specification. Through a stepby-step procedure of alternating delay measurement and network readjustment, a line equalization task which formerly consumed days or weeks of time can be accomplished with the aid of the invention in a matter of hours.

Moreover, if the carrier frequency is swept repetitively through the frequency range of interest, at a uniform rate, while the delay meter current is applied to the vertical pair of plates of a cathode ray oscilloscope which has its horizontal pair of plates enregized by a second signal derived form and proportional to the sweeping carrier frequency, then the oscilloscope will delineate a plot of signal delay against frequency.

Such displays are illustrated in FIGS. 9 and 10 and constitute an instantaneous and direct picture of the delay characteristic of the line through the frequency range of interest and are suitable for observing the effect of compensating equipment adjustments as they are being made, and without further analysis being required.

In devices intended for measurement or other purposes which employ the principle of comparing time or phase relationships of frequencies, the stability of the two frequencies under comparison is highly important. Preferably the peaks of the waves would be chosen as the comparison points but they are difiicult to sense accurately. Instead it is customary to use as comparison points the instant of cross-over of the zero axis by the respective waves. These points again are subject to erraticities unless special precautions are taken to avoid them. These difficulties are overcome in the present invention by expedients now to be described.

In the limiter and pulse former of FIG. 17, which comprise the blocks K and M of FIG. 13, the early stages clip the incoming wave at levels lower than that chosen for the subsequent stages. By this expedient the received wave is amplified in each of the stages beyond it clipping level so that a substantial measure of wave squaring is accomplished in each stage whereby at the output of the final stage a vertically square wave, symmetrical with respect to the zero axis, is achieved. By thus maintaining symmetry the phenomenon termed herein axis shift is almost entirely eliminated. Elimination of axis shift is furthered by the choice of the comparison frequency presented to the delay measuring flip-flop. Instead of '25 pulses per second corresponding directly to the frequency of the received envelope, 50 pulses per second are used for comparison with a 50 pulse per second reference rate generated by the fork at the receiver.

In explanation, it is apparent that if the amplitude of the 25 cycle wave shifts upward or downward with respect to its zero axis :as a consequence of changing line attenuation, the positive and negative half wave lose their symmetry and become unequal in length. The time position of any individual cross-over, therefore, would not be representative of the actual line delay. However, by sen-sing each cross-over the respective long and short half waves are averaged to give an effective reference point truly repersentative of the arrival time of the wave. This reference point is in effect equivalent to the peak points of the positive and negative half waves. By this expedient the final delay readings are uninfluenced by amplitude changes of as much as 40 db.

Little has been said up to this point about the facility included in the measuring instrument for the measurement of frequency. Since frequency comprises the abscissa in the customary delineation of delay curves, an accurate, automatic and continuous means of measuring frequency must accompanying the similar measuring facility for time delay. For this purpose, I include a frequency indicating device, preferably of the counter type, which responds to the carrier component of the modulated received wave. Hence, at any instant, the operator can properly coordinate measurements of time delay with frequency. The frequency meter circuit is extended to the horizontal pair of plates of an oscilloscope whose vertical pair of plates is connected to the delay meter. With this arrangement, a continuous delineation of the delayfrequency measurement of a line or device may be presented, and this is particularly helpful in the process of delay equalization It is Well known, of course, that nonlinearity of attenuation with frequency is also a cause of distortion of signals. In order to measure the frequency-attenuation characteristic, it is a fairly simple matter to connect a level meter in the receiving circuit in advance of the limiters and to observe the level of the incoming wave as its frequency is swept over the region of interest. In the same manner as outlined for recording delay measurement, the level meter may be connected to the vertical pair of plates of an oscilloscope for delineating frequency-attenuation curves.

The measuring instrument, therefore, lends itself uniquely for use in programs for coordinated correction of the delay and attenuation factors principally responsible for causing distortion in signals. Particularly adapted for this purpose are the compensating networks, to be described later, designed to correct nonlinearity in both delay and attenuation.

- Delay compensating networks When a transmission line network or device has been found to possess irregular delay over its useful frequency range through measurement with a delay measuring instrument such as described in the foregoing, the distortion to signals which follows may be eliminated through the addition of delay compensating networks to the system under measurement. The purpose, of course, is to linearize a nonlinear phase shift characteristic such as has been indicated in FIG 2. When corrected, the delay and phase shift curves of this figure will resemble those of FIG. 1 except that the phase shift curves probably will not pass through the origin. Since it is not possible to introduce negative delay, the compensation is effected by introducing positive delay in suitable amounts at those frequencies which suffer least delay when transmitted over the line or network.

While there are many types of delay compensating networks, I have found that a. species of bridge type network which may be used singly, or in tandem, but preferably with a gain element such as a vacuum tube or semiconductor amplifying device, interspersed between sections for the purpose of recouping the attenuation loss imposed by the networks. Each network is effective over a relatively narrow range of frequency and it is convenient to employ as many as a dozen or more such networks in series to cover a frequency band of several kilocycles. Each network introduces a resonance like curve in the delay characteristic so that, when properly spaced, a series of such curves tend to merge together to form a final relatively fiat overall compensated characteristic. The residual peaks of the resonance like curves may readily be kept within the required delay tolerances for the systerm.

The bridge networks comprise a pair of ratio arms usually fixed and usually of nonunity ratio, and a second pair of arms, at least one of which includes one or more reactances, usually a resonant circuit and possibly with a damping resistance, while the fourth arm usually comprises a variable resistor. A large number of permutations of these elements are possible depending upon the manner in which the network is to be used, the nature of the circuit characteristic being compensated, the terminal equipment and the type of associated amplifying device. These practical considerations favor a few generalized types, three of which are illustrated at (a), (b) and (c) in FIG. 5. In each of these, an impedance transformation between input and output occurs so that the networks complement the opposite impedance transformations which occur in most amplifying devices thus permitting convenient coupling to preceding or following amplifying devices.

In FIG. (a) is shown a constant voltage network arranged for low impedance input and high impedance output. As indicated, the input voltage is applied by means of a transformer bearing an intermediate tap to provide a nonunity ratio input designated by the factor K. Generalized impedance arms Z, and Z are shown in the opposed bridge arms. FIG. 5 (b) presents a constant current type of network embracing a high impedance input and a low impedance output. The input current may be applied from a suitable transformer, or by direct coupling, as may be convenient. In this network, all four arms of the bridge can be made adjustable in pairs. In the constant current network of FIG. 5(0) a high impedance input and a low impedance output are again characteristic. This network includes two fixed resistance arms bearing the ratio R /R =K.

In each of the above networks, connections to ground have been indicated, but it should be understood that these are entirely optional depending upon the requirements of the associated circuitry.

In order to bring out the unique performance of certain circuit combinations when embodied into these network configurations, an analytical presentation follows. In the network of FIG. 5(a) the output-to-input transfer ratio is ofthe form t v.1 'r- J I 'E "Z fl-Z (1 where Z,, and 2 are functions of w, Zr and the param; eters of the network elements shown at Z and Z while K is a proportionality facton The factor K may bereal or complex, but only real numbers will be considered here. I

For K-= 3, Equation-.1 can be satisfied by theuse .of a resistance for Z,, and a resistance R- andreactance X in parallel for 2,, so that 1 x43 Efe -R where M is a constant. Of these, a very useful form occurs when M=1 and R =R =R in which case the output-input transfer ratio is 3jRX(R X and A simple and practical network type is shown in FIG. 6, which is analogous to FIG. 5(a) previously considered. In FIG. 6, from Equation 4 above i so that for a given frequency and fixed value of C a phase shift reaching approximately from 0 to 360 is obtained with constant amplitude throughout the range merely by simultaneously varying the two equal resistors R.

A network according to FIG. 7 has been found highly useful. In FIG. 7, 'R is used for Z,,, and for Z the three-element parallel combination L, C, R is inserted. The K factor is provided by the ratio R /R If R R are maintained equal throughout their range of adjustment, the amplitude characteristic will remain constant, while from Equation 2 the delay characteristic will vary according to sistor R In this case, the output-input transfer ratio and delay can readily be shown to be B 2 tan- If R is larger than R the amplitude characteristic, from Equation 7, will be peaked upward in the vicinity of the resonant frequency w If R is smaller than R it will be peaked downward.

Reference has previously been made to the ratio factor K and the preference for the values of 3 and 5 for this factor. It has just been proved that it is characteristic of the networks of FIGS. 5 and 6 that by choice of the parameters of the impedances Z and Z, a value of K of either 3 or 5 may be achieved. It is further characteristic of networks with K equal to 3 that the delay and amplitude characteristic may be adjusted separately and independently. This property, unique in the networks described herein, is of extraordinary practical importance. Its origin rests upon the phase shift Equation 8 previously asserted where it is apparent that phase shift comprises the sum of two factors each of which differs only in that the numerator is in the one case the sum of the two resistances R and R and in the other case the difference of these two resistances. Hence while adjusting the delay characteristic of the network by varying R and R if the resistors are maintained equal the delay characteristic will be dependent upon their common value while the amplitude characteristic will remain constant for all settings. If, in addition, it is desired to vary the amplitude characteristic this can be accomplished by offsetting resistor R with respect to resistor R and the delay will remain substantially constant inasmuch as while one term of Equation 8 declines, the other will rise in compensating degree.

The network of FIG. 7 has been found to be a particularly useful form of K=3 network. The K factor is obtained by the ratio of the resistors R /R where the value of the resistance of R is in fact the combined resistance of R and the resistance of the paralleling coil L.

The values of K3 or 5 are somewhat unique but the utility of the networks described herein is not limited precisely thereto. It is of considerable practical convenience if, for example, the two adjustable resistors of FIGS. 6 and 7 can be ganged together for simultaneous and equal adjustment, However, K may be varied within a moderate range to accommodate inequalities between the two adjustable resistors, introduced for purposes of attenuation adjustment, with only minor limitation in range of usefulness of the networks. The relationship between R /R and K, or R /R in FIG. 7 may be developed from Equation 7 if the symbol K is substituted for the numeric 3 therein. The right-hand term of Equation 7 is in the form A+jB for an all-pass network wherein the real and imaginary terms in the numerator are, respectively, equal to those in the denominator. Then, equating the imaginary terms it is found that:

For K=3 or 5, attenuation remains small and constant, even when using small and moderate quality coils for the inductors. While the ratios may vary by as much as 25% without severe impairment, departure beyond this amount is likely to introduce attenuation loss beyond that which can readily be regained by a single stage of amplification.

Thus with these K 3 networks it is possible to adjust the delay characteristic over a wide range and the amplitude characteristic over a somewhat restricted range essentially independent of each other merely by adjusting two resistors while maintaining a proper relationship between them in accordance with Equations 7 and 8. Practically, it is usually necessary to change only R from equality with R to change from a flat amplitude characteristic to a peaked characteristic in the vicinity of (n Such convenience is not possible with passive networks which usually require separate networks for amplitude and delay distortion correction since each alteration of the amplitude characteristic of a purely amplitude corrective network introduces some delay distortion which in turn must be offset in the delay corrective network. For this reason, amplitude distortion correction is usually made first. With the K=3 active, network, both processes proceed simultaneously using only a single network.

A network type which lends itself uniquely to the K :5 ratio is that of FIG. 6. In this simple configuration simultaneous adjustment of the ganged resistors R is obtained through a range of 0 to 360 while maintaining constant amplitude. The K=5 networks permit wider variations of delay but do not allow as extensive manipulation of attenuation values as is the case with the K=3 networks. In general, the latter type is more useful.

FIGS. 8, 9 and 10 are illustrative of the type of characteristic, both delay and amplitude, obtainable with the active type networks. FIG. 8 represents photographs of two oscilloscope faces, the upper one presenting the delay characteristic obtainable with an active network such as that of FIG. 7, while the lower face is representative of the amplitude correction which may be inserted at the same time by the same network. The delay curves are displaced vertically with respect to each other in order to illustrate the type of peaking which may be obtained. The amplitude curves all correspond to the intermediate delay curve and, as shown, may be either fiat, or peaked upward or downward.

These networks are not only useful for correcting regular or random delay and amplitude aberrations but they may also be employed for simulating various amplitude distributions at an essentially constant delay. For example, a filter to accommodate vestigial sideband operation, usually subject to serious delay distortion, may be simulated with constant delay throughout the effective bandwidth. FIGS. 9 and 10 illustrate similar oscilloscope photographs representing the delay and amplitude characteristics of a loaded cable circuit having the approximate bandwidth of 300 cycles to 3500 cycles, before, and after correcting, respectively, by means of the compensating networks of the invention. In this case, a series of active networks in tandem were individually adjusted while observing the effect of the adjustments by means of the hereinbefore described measuring system. It Will be noted that a relatively flat delay characteristic over the complete range has been realized. By means of the same networks, a relatively linear but, in this case, slightly rising attenuation characteristic has been obtained. The results shown in these two figures are but illustrative and it should be realized that by proper network adjustment, a great variety of characteristics may be obtained by the use of a single type of network. It is obvious also that since the overall curves comprise an addition of the plurality of peaked component curves, then smoothness of the final curves will be somewhat proportional to the total number of network sections employed.

v OPERATION OF DELAY MEASURING INSTRUMENT FIG. 13 presents an overall block diagram of the measuring system as arranged for making delay measurements on a line where the two terminals are isolated from each other. The transmitter section comprises essentially a source of variable carrier frequency, a highly stable source of a relatively lower'modulating envelope frequency, and a modulator. The receiver includes, essentially, a demodulator for the envelope modulated incoming carrier frequency, together with associated amplifying, limiting and pulse forming circuits for furnishing to the flip-flop comparison device a series of pulses representative of the time of arrival of the envelope waves. For comparison, or reference, purposes in the flip-flop, a similar series of pulses is obtained from a frequency standard and associated apparatus, parts of which may be identical with that employed at the transmitter. The output from the flip-flop may operate directly a coarse delay meter for indicating the envelope delay at the carrier frequency under measurement. At the same time, a frequency meter, energized from the receiving amplifier, provides a continuous indication of the momentary carrier frequency, while a level meter included at the output of the demodulator presents a continuous indication of received carrier frequency level. The coarse delay meter is particularly useful for measuring wide range areas of delay and for general monitoring purposes. For more sensitive indications, a high sensitivity meter, with an appropriate amplifier, is added. For oscilloscopic presentation, the operating potentials for the high sensitivity delay meter and the level meter may be connected alternatively to the vertical deflection plates of the oscilloscope, while the frequency meter potential is connected to the horizontal plates. A more detailed description of the overall system of FIG. 13 follows.

From a tuning fork frequency standard A, a frequency of 200 cycles per second is fed to an amplifier and pulse former C producing a continuous pulse train having a repetition rate accurately maintained at 200 pulses per second. This enters an aperiodic divided chain D of three stages composed of bistable circuits, and emerges as a train of rectangular waves having a repetition rate of 25 pulses per second. In an integrating amplifier E, these are converted to triangular waves, which, when applied to a parallel circuit resonant at 25 cycles per second, cause the production of a substantially sinusoidal modulating voltage at 25 cycles per second having a high order of frequency accuracy and stability.

Simultaneously with the foregoing, a beat frequency oscillator F produces a carrier signal of relatively higher frequency, and the fequency of the variable frequency oscillator thereof is swept repetitively through a selected frequency range by a motor, to comprise in its entirety a sweep oscillator, for providing a carrier frequency swept linearly over a selected frequency range to be measured.

The 25-cycle voltage and the swept carrier frequency signal are applied to a modulator G so arranged as to provide an output amplitude modulated to the extent of about 50% but to substantially suppress the 25-cycle modulating voltage and all other modulation products except the carrier and first order sidebands. The output signal comprising the carrier and upper and lower sidebands is then applied through a level adjusting and impedance matching attenuator pad H to the transmission line L which it is desired to investigate.

At the distant end of the line the received 25-cycle modulated carrier is applied to an amplifier I which feeds a demodulator and sharp band pass filter J for recovery of the 25-cycle modulating signal, which, due to its passa-ge along the line in envelope form, has suffered a delay identical to that experienced by the carrier frequency under consideration, but more readily measured, as previously explained. Passing to a limiter K, noise and level disturbances are therein eliminated from the signal and it is applied to a pulse former M for the generation of a succession of reception pulses having a pulse rate of 50. .per second with their occurrence precisely timed at a selected and distinctive portion of the regenerated modulated wave, in this case the zeroor cross-over point. I

Concurrently with the above receiver operation, a local tuning fork oscillator B adjusted closely but not necessarily precisely to 200 cycles per second supplies an amplifier N, divider O and pulse forming circuit P to provide a reference standard signal of 50 pulses per second, which, together with the aforesaid reception pulses, are supplied to flip-flop circuit Q. The generator of the receiving-end reference pulses may be identical with the 25-cycle generator as used in the transmitter except that the SO-cycle frequency is procured from an earlier stage of the divider. In fact, if both terminals of the device under measurement were available at one location, a single frequency standard could be used for both the transmitted and reference waves. This facility can lead to economy of equipment for certain measuring assignments, and also influences the packaging of the associated equipment assemblies and their power supplies.

The operation of the flip-flop to compare the time occurrence of the pulses derived from the incoming envelope wave with locally generated reference pulses, both at the rate of 50 per second, to produce an output voltage for the coarse delay meter indicative of envelope delay has already been explained in detail in connection with FIG. 4 and need not be repeated at this point.

For more accurate measurement of delay, this flipflop voltage is also applied first to an amplifier S, and thence to a high sensitivity delay meter T having means for electrical zero suppression and provided also with a scale selective switching network. Carrier frequency is measured by passing received signals from the receiving amplifier I through a limiting circuit and integrator U to remove the effects of modulation and to provide a voltage proportional to carrier frequency, which is applied to a voltmeter V for indication of frequency. A carrier frequency level meter Z may be connected at any suitable point as at the demodulator J.

For visual display the carrier frequency voltage may be applied to the X-axis amplifier of a cathode ray oscilloscope W, and when the aforesaid delay signal is applied to the Y-axis, a presentation of delay time as a function of carrier frequency, in Cartesian coordinates, appears on the luminous screen thereof. In the same manner, curves of level versus carrier frequency may be portrayed on the same or a second oscilloscope.

This instrument measures the first derivative d/3/dw of a signal envelope, which in the present case is composed exclusively of a carrier frequency and only two symmetrically spaced side frequencies. It has been proved that for such a modulated wave dB/dw is a measure of the tendency of a line to produce delay distortion in facsimile or other signals of like character, and further, in the distortionless case only, that dB/dw of the modulated envelope is equal to the phase delay fi/w of the above mentioned carrier frequency. Hence for all practical purposes the dp/dw oscilloscope tracing represents the relative delay characteristic of the line under test over the swept frequency range, and corrections designed to render this characteristic constant over the desired frequency range will free the line of the tendency to produce phase distortion in signals.

As the oscilloscope spot is impelled across the screen by the gradually sweeping carrier frequency it in effect traces a continuous series of points, the heading of each successive point upward or downward being indicative of the rate of change of envelope delay, with respect to the change in carrier frequency, encountered in the line since the point last precedingwas recorded. If the scope beam was interrupted at frequent but regular intervals the resulting curve would correspond to a point-by-point curve taken by hand methods. At a rapid rate of sweep the effective fictitious recurring frequency of the points 17 is relatively coarse. At the slow rate the point frequency becomes fine and the delineation of curves having sharp peaks is feasible.

DETAILED DESCRIPTION OF THE CIRCUITS The detailed operation of the circuits, beginning with FIG. 14 comprising the transmitting portion of the measuring equipment, is as follows:

Transmitting portion A ZOO-cycle per second tuning fork standard of frequency 3, of any well-known but very high stability type, supplies a signal to an amplifier and pulse forming circuit 4, which produces one pulse per cycle to drive a three-stage frequency dividing and square wave producing generator 5, in a manner usual in the art. Alternatively, the frequency standard of extraordinary stability with associated divider system, as described in my copending US. application Ser. No. 727,354, filed Apr. 9, 1958, now Patent No. 2,998,576, and indicated schematically herein in FIG. 40, may be substituted to advantage for this modulating frequency source. Square waves from the producer 5, indicated as a signal balanced to ground from the output plates thereof, are applied through the coupling condensers 6, to balanced integrating circuits comprising series resistors 7 and shunt condensers 8 to ground, the triangular wave produced thereby being applied to the parallel resonant circuit comprising condenser 9 and inductance it), which is tuned to resonate at the fundamental frequency of the said triangular wave or 25 cycles per second, to thereby produce a 25-cycle sine wave. This 25-cycle sine wave is then applied to the grids 11 of an amplifying duplex vacuum tube l2 under control of grid leaks l3 and common cathode bias resistor 14. The amplified output from the push-pull connected plates 15 is fed to transformer 16 whence a portion thereof, from the voltage divider comprising the resistors 17 connected across the output of the said transformer, is applied in balanced fashion to the two grids 19 of the duplex triode modulating tube 18 by introducing it at the center tap of the modulation transformer 20'. Fixed bias for the cathodes 21 is provided by and the balance thereof to ground is achieved with the aid of adjustable potentiometer 22.

The purity and identity of the -cycle sinusoidal waves as generated and the 25-cycle envelope as transmitted herein importantly influence the accuracy of delay measurement. Hence this modulator is of a design which suppresses the modulating frequency and the higher order modulation components to a very high degree so that the 50% modulated output wave consists virtually solely of the carrier wave and the two first order sidebands spaced, respectively, 25 cycles above and below the carrier. While the modulator employing the twin triode tube 13, with circuitry as shown, meets these requirements satisfactorily, an alternative employing two remote cutoff pentodes instead of the twin triode 18 and having a much higher plate resistance also permits modulation at the high level desired without the introduction of significant harmonic distortion into the modulated wave. In this latter case, each half of the primary winding of the line transformer should preferably be shunted by resistances of the same order as the transformer primary impedance so that changes in plate impedance of the pentodes would have but small effect on the ultimate output impedance and hence provide a more constant output level to line.

The beat frequency oscillator 49 comprises two beating oscillators 23 and 33 each having a frequency in the neighborhood of 150 kc. and which may be virtually identical except that the tuning condenser of the tank circuit of oscillator 23 is arranged to be varied repetitively by a motor to provide the desired frequency sweeping action, while in oscillator 33 this same condenser, while normally fixed, may manually be given a number of different settings to determine the position of the swept band.

The schematic circuitry of the variable oscillator 23 is shown in FIG. 11. This oscillator, of known type, has extraordinary frequency stability and constancy of output level over the sweep range. A twin triode tube 50 is employed and the generated frequency is determined by the tank circuit Sll embracing an inductor 52 and tuning condensers 53 and 54. Condenser 53 is relatively large so that as the smaller condenser 54 is varied through its range, the sweep-frequency rate will be substantially linear. Condenser 54 is not only variable in capacity, but its maximum capacity may be given different settings so that the swept band may be located in different areas of the total spectrum over which measurements are being made. These settings are accomplished by mechanically varying the spacing between condenser plates. The sweep has a repetitive triangular pattern such that the output frequency increases to its upper limit on one-half of the revolution of the rotor and then descends at the same rate to the bottom frequency. This avoids sudden large changes in the sweep frequency with consequent disturbance to the measurement section of the instrument.

The low end of the sweep range is set by adjustment of the analogous condenser 54 in the fixed frequency oscillator 33 so as to establish a minimum frequency separation between the two oscillators thus providing an independent means for establishing the low end of the sweep range. The high end of the sweep range depends upon maximum capacity, that is, the distance setting between the plates of the sweep condenser 54 in the sweeping oscillator. Thus, both ends of the sweep range may be set quickly and independently wherever desired without the need of a trial and error process. Output of the variable oscillator is then fed to the secondary center tap of transformer 30 of the modulator via series condenser 26 and shunt resistor 27, while the output of the fixed oscillator 33 is fed to the primary of transformer 30 of the same modulator.

The signal from oscillator 23 is applied symmetrically to the center tap of the secondary of modulation transformer 36 which is shunted by the matched resistors 28, and thus to the grids 31 of the duplex triode modulator tube 32. Variable oscillator frequency is thus mixed in the tube 32 with constant frequency oscillations from fixed oscillator 33, applied by transformer 30 to the grids 31 of the tube 32. The output thereof is taken from the plates 34, while appropriate adjustment of balancing potentiometer 36 provides for differential trimming of the cathode bias developed in resistor 37 to adjust balance of the two halves of tube 32. Series inductors 38 and shunt condensers 39 working into shunt resistor 41 provide low pass filtering for the modulated output of tube 32, and the balanced series resonant circuit comprising condensers 42 and inductors 43 further suppress the two mixing frequencies and unwanted modulation products to insure that only a pure carrier frequency signal, swept as desired, will be applied to the matching transformer 44. Such signal is applied through the modulation transformer 20 to the grids 19 of the modulator tube 18, and the output thereof is modulated by the aforementioned modulation frequency signal from transformer 16.

The resultant signal, comprising the periodically swept carrier, with associated first order sidebands, therefore, furnishes for ultimate use at the receiving end of the system a pair of sideband frequencies with fixed close spacing for the determination of delay while the intermediate spaced carrier indicates the instant frequency at which measurements are being made, all as will appear hereinafter. This signal is then fed from the plates 46 through the matching transformer 47 to the conventional attenuator and line termination pad 48, and thence to the transmission line under investigation.

Throughout the equipment, vacuum tube cathodes are rendered emissive by heaters having conventional circuitry, which for the sake of clarity is not shown on the drawings, and the positive pole of a grounded, pref- 

